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Not sure if I had asked this before and I couldn't find anything here....so at the risk of sounding stupid, here goes.....I am getting Class D audio amps for repair now......having just a basic 2 channel scope with a 20 Mz bandwidth should be ample....I have an Ampeg PF500 here now and found that one of the mosfets were shorted between all three pins.....I removed both fets and did some resistance checks and could not find anything out of wack and there are no signs of other damaged components....I want to scope the output of the gate driver....as these circuits are referenced to the negative voltage rail, what is the safest way to measure the gate drives with my scope so that I don't blow anything up......I obiviously can't connect scope ground to a voltage rail and I don't have a differential probe..I have two single ended probes....I just want to make sure that both gate drive signals are ok to rule out small support components being weakened from the shorted fet.....so if I scoped the gate drive waveforms right at the gate pads of the fets,...and the waveforms looked ok, then I should be able to install two new fets....just looking for some sound and safe advice.....and I guess this would apply to any digital amp??
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If I understand correctly all you need to do is connect scope ground to amp ground and use AC coupling with scope input. This assumes that there is no signal on the rails.
Mind to post schematic?Last edited by Helmholtz; 11-21-2020, 06:16 PM.- Own Opinions Only - -
These are the only parts of the schematic I've seen (2 pages). As far as I know, the only place you have to be careful about your ground is on the primary side of the SMPS.Originally posted by Enzo
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Trying to copy the relative page but can't get it to work.....I'll have to try something else.....the manual is 37 pages and I am not allowed to post that due to non disclosure agreements....Comment
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For class D amp output, my guess is your concern is the high side driver as the MOSFET source pin is connected to the the output. Not a perfect method, but should give you at least good information, is to use two probes. CH1 goes to the gate and CH2 goes to the source, ground clips go to 0V. You then use the arithmetic function of you scope to display CH1-CH2.
A good differential probe costs serious money and will hardly ever get used.Last edited by nickb; 11-22-2020, 03:16 PM.Experience is something you get, just after you really needed it.Comment
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Yes but isn't the low side fet connected to the - rail.....I was thinking that you would not need to connect scope probe grounds to the circuit as the amp is connected to mains and the scope is also connected to mains....and yes...those differential probes do cost a lot of money...definately out of my price range for sure.....Originally posted by nickb View PostComment
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Well you didn't give a schematic so i had to guess what the topology was. If the 0V reference is not the same as that for the low side FET as you say, them there is no difference to the method, you just hook up the the low side FET instead, CH1 to gate, CH2 to source.Originally posted by bsco View Post
The problem with this method (in both cases) comes from the small difference in gains between the two channels and also that the voltage common to both probes is large. Suppose the gate voltage is 5V, the mid point swing is +/- 50V and the gain error is 0.01 ( i.e. 1%). Then you will see on the scope a 1% error in the wanted difference signal but also 1% of the common voltage difference i.e. 1V. 1V is large compared to the 5V gate voltage. So, for best results minimize the gain difference between the channels before you start. You can do this by connecting both probes to the common point with the +/- 50V swing and using the fine gain control on the scope to get minimum wiggle on the scope trace. Proper probe compensation will help too. Ensure both probe clips go to the same point.Experience is something you get, just after you really needed it.Comment
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WHAT MOSFETS? The ones in the SMPS? or the ones in the power amps?
If you mean in the SMPS, that is on the primary side and is DANGEROUS. Use an isolation transformer for the unit under test. That will eliminate the direct mains connection.
If you are talking about the powr amps, they are on the secondary side, and they have no idea what sort of power supply was used. On that side ground is ground.Education is what you're left with after you have forgotten what you have learned.Comment
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Enzo - we are talking about Class D power amplifiers, not SMPS.Experience is something you get, just after you really needed it.Comment
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it's true. what a drag, too.Originally posted by nickb View Post
I think it's a tool worth having. But it's one of those things where I will need to be patient, and wait for the right used one to come along. A new one would start at what I spent on my new scope, and could easily end up doubling the cost .Last edited by SoulFetish; 11-22-2020, 04:10 PM.If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.Comment
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The power amp mosfets...and I always use an isolation transformer when servicing anything that connects to the line....Originally posted by Enzo View PostComment
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see schem. attached:Originally posted by Enzo
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Thanks for doing that..I couldn't get it to work for me,.....Originally posted by g1 View Post
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Schematic is posted..g1 did it for me as I couldn't get it to work.....Originally posted by Helmholtz View PostComment
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You can make your own probe for low cost: https://hackaday.io/project/169390-a...erential-probe
Methinks this is a nice idea. One will be appearing on my bench soon
Experience is something you get, just after you really needed it.Comment
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